LOW-FLUENCE LASER TARGET COUPLING

Low-fluence laser interaction with metallic target is investigated in a wide range of irradiance, from 5 x 10(6) to 10(11) W/cm2, with 4- and 30-ns laser-pulse durations and 1.06-mu-m wavelength. Edge effects are avoided by means of large surfaces of irradiation (5 cm2). Specific diagnostics such as a ballistic pendulum and a piezoelectric quartz gauge are developed to measure the total momentum imparted to the target, as well as the temporal evolution of the stress at the rear side of the target. Specular reflection and side-scattering of the laser light from the target are also measured. The behavior of 2024 aluminum alloy is particularly studied. The results are interpreted in terms of liquid-vapor transition and change in laser absorption. In light of experimental data, the theoretical analysis of surface absorption is reviewed, showing the dependence on metal temperature and laser wavelength. The thermal and mechanical coupling is also calculated. Numerical simulations are performed with a 1-D Lagrangian hydrodynamic code including modifications for low-pressure regimes. The multiphase equation of state is extended to take into account negative pressures. An elastoplastic module is introduced into the code. A quite good agreement is obtained with the measurements of the stress and of the mechanical coupling coefficient.